diff --git a/autoload/ale/c.vim b/autoload/ale/c.vim
index b9f94399..f6ad7deb 100644
--- a/autoload/ale/c.vim
+++ b/autoload/ale/c.vim
@@ -1,6 +1,8 @@
 " Author: gagbo <gagbobada@gmail.com>, w0rp <devw0rp@gmail.com>
 " Description: Functions for integrating with C-family linters.
 
+let s:sep = has('win32') ? '\' : '/'
+
 function! ale#c#FindProjectRoot(buffer) abort
     for l:project_filename in ['.git/HEAD', 'configure', 'Makefile', 'CMakeLists.txt']
         let l:full_path = ale#path#FindNearestFile(a:buffer, l:project_filename)
@@ -47,7 +49,7 @@ function! ale#c#FindLocalHeaderPaths(buffer) abort
 
     " If we find an 'include' directory in the project root, then use that.
     if isdirectory(l:project_root . '/include')
-        return [ale#path#Simplify(l:project_root . '/include')]
+        return [ale#path#Simplify(l:project_root . s:sep . 'include')]
     endif
 
     return []
@@ -79,7 +81,7 @@ let g:ale_c_build_dir_names = get(g:, 'ale_c_build_dir_names', [
 function! ale#c#FindCompileCommands(buffer) abort
     for l:path in ale#path#Upwards(expand('#' . a:buffer . ':p:h'))
         for l:dirname in ale#Var(a:buffer, 'c_build_dir_names')
-            let l:c_build_dir = l:path . '/' . l:dirname
+            let l:c_build_dir = l:path . s:sep . l:dirname
 
             if filereadable(l:c_build_dir . '/compile_commands.json')
                 return l:c_build_dir
diff --git a/autoload/ale/path.vim b/autoload/ale/path.vim
index 57e607a5..b0f4dca6 100644
--- a/autoload/ale/path.vim
+++ b/autoload/ale/path.vim
@@ -1,14 +1,23 @@
 " Author: w0rp <devw0rp@gmail.com>
 " Description: Functions for working with paths in the filesystem.
 
+" simplify a path, and fix annoying issues with paths on Windows.
+"
+" Forward slashes are changed to back slashes so path equality works better.
+"
+" Paths starting with more than one forward slash are changed to only one
+" forward slash, to prevent the paths being treated as special MSYS paths.
 function! ale#path#Simplify(path) abort
-    " //foo is turned into /foo to stop Windows doing stupid things with
-    " search paths.
-    return substitute(simplify(a:path), '^//\+', '/', 'g') " no-custom-checks
+    if has('unix')
+        return substitute(simplify(a:path), '^//\+', '/', 'g') " no-custom-checks
+    endif
+
+    let l:win_path = substitute(a:path, '/', '\\', 'g')
+
+    return substitute(simplify(l:win_path), '^\\\+', '\', 'g') " no-custom-checks
 endfunction
 
 " This function is mainly used for testing.
-" Simplify() a path, and change forward slashes to back slashes on Windows.
 "
 " If an additional 'add_drive' argument is given, the current drive letter
 " will be prefixed to any absolute paths on Windows.
@@ -16,8 +25,6 @@ function! ale#path#Winify(path, ...) abort
     let l:new_path = ale#path#Simplify(a:path)
 
     if has('win32')
-        let l:new_path = substitute(l:new_path, '/', '\\', 'g')
-
         " Add a drive letter to \foo\bar paths, if needed.
         if a:0 && a:1 is# 'add_drive' && l:new_path[:0] is# '\'
             let l:new_path = fnamemodify('.', ':p')[:1] . l:new_path
@@ -86,6 +93,10 @@ endfunction
 
 " Return 1 if a path is an absolute path.
 function! ale#path#IsAbsolute(filename) abort
+    if has('win32') && a:filename[:0] is# '\'
+        return 1
+    endif
+
     " Check for /foo and C:\foo, etc.
     return a:filename[:0] is# '/' || a:filename[1:2] is# ':\'
 endfunction
@@ -103,7 +114,7 @@ endfunction
 " directory, return the absolute path to the file.
 function! ale#path#GetAbsPath(base_directory, filename) abort
     if ale#path#IsAbsolute(a:filename)
-        return a:filename
+        return ale#path#Simplify(a:filename)
     endif
 
     let l:sep = has('win32') ? '\' : '/'
@@ -145,8 +156,8 @@ endfunction
 
 " Given a path, return every component of the path, moving upwards.
 function! ale#path#Upwards(path) abort
-    let l:pattern = ale#Has('win32') ? '\v/+|\\+' : '\v/+'
-    let l:sep = ale#Has('win32') ? '\' : '/'
+    let l:pattern = has('win32') ? '\v/+|\\+' : '\v/+'
+    let l:sep = has('win32') ? '\' : '/'
     let l:parts = split(ale#path#Simplify(a:path), l:pattern)
     let l:path_list = []
 
@@ -155,7 +166,7 @@ function! ale#path#Upwards(path) abort
         let l:parts = l:parts[:-2]
     endwhile
 
-    if ale#Has('win32') && a:path =~# '^[a-zA-z]:\'
+    if has('win32') && a:path =~# '^[a-zA-z]:\'
         " Add \ to C: for C:\, etc.
         let l:path_list[-1] .= '\'
     elseif a:path[0] is# '/'
diff --git a/test/handler/test_javac_handler.vader b/test/handler/test_javac_handler.vader
index 3997b42c..6189e6e1 100644
--- a/test/handler/test_javac_handler.vader
+++ b/test/handler/test_javac_handler.vader
@@ -12,33 +12,33 @@ Execute(The javac handler should handle cannot find symbol errors):
   AssertEqual
   \ [
   \   {
-  \     'filename': '/tmp/vLPr4Q5/33/foo.java',
+  \     'filename': ale#path#Simplify('/tmp/vLPr4Q5/33/foo.java'),
   \     'lnum': 1,
   \     'text': 'error: some error',
   \     'type': 'E',
   \   },
   \   {
-  \     'filename': '/tmp/vLPr4Q5/33/foo.java',
+  \     'filename': ale#path#Simplify('/tmp/vLPr4Q5/33/foo.java'),
   \     'lnum': 2,
   \     'col': 5,
   \     'text': 'error: cannot find symbol: BadName',
   \     'type': 'E',
   \   },
   \   {
-  \     'filename': '/tmp/vLPr4Q5/33/foo.java',
+  \     'filename': ale#path#Simplify('/tmp/vLPr4Q5/33/foo.java'),
   \     'lnum': 34,
   \     'col': 5,
   \     'text': 'error: cannot find symbol: BadName2',
   \     'type': 'E',
   \   },
   \   {
-  \     'filename': '/tmp/vLPr4Q5/33/foo.java',
+  \     'filename': ale#path#Simplify('/tmp/vLPr4Q5/33/foo.java'),
   \     'lnum': 37,
   \     'text': 'warning: some warning',
   \     'type': 'W',
   \   },
   \   {
-  \     'filename': '/tmp/vLPr4Q5/33/foo.java',
+  \     'filename': ale#path#Simplify('/tmp/vLPr4Q5/33/foo.java'),
   \     'lnum': 42,
   \     'col': 11,
   \     'text': 'error: cannot find symbol: bar()',
diff --git a/test/handler/test_tslint_handler.vader b/test/handler/test_tslint_handler.vader
index bbaef837..8d263efa 100644
--- a/test/handler/test_tslint_handler.vader
+++ b/test/handler/test_tslint_handler.vader
@@ -287,9 +287,9 @@ Execute(The tslint handler should not report no-implicit-dependencies errors):
 Execute(The tslint handler should set filename keys for temporary files):
   " The temporay filename below is hacked into being a relative path so we can
   " test that we resolve the temporary filename first.
-  let b:relative_to_root = substitute(expand('%:p'), '\v[^/\\]*([/\\])[^/\\]*', has('win32') ? '..\' : '../', 'g')
+  let b:relative_to_root = substitute(expand('%:p'), '\v[^/\\]*([/\\])[^/\\]*', '../', 'g')
   let b:tempname_suffix = substitute(tempname(), '^\v([A-Z]:)?[/\\]', '', '')
-  let b:relative_tempname = b:relative_to_root . b:tempname_suffix
+  let b:relative_tempname = substitute(b:relative_to_root . b:tempname_suffix, '\\', '/', 'g')
 
   AssertEqual
   \ [
diff --git a/test/test_c_import_paths.vader b/test/test_c_import_paths.vader
index af185eae..21e49a3a 100644
--- a/test/test_c_import_paths.vader
+++ b/test/test_c_import_paths.vader
@@ -40,7 +40,7 @@ Execute(The C GCC handler should include 'include' directories for projects with
   \ ale#Escape('gcc')
   \   . ' -S -x c -fsyntax-only '
   \   . '-iquote ' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/makefile_project/subdir')) . ' '
-  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/makefile_project') . '/include') . ' '
+  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/makefile_project/include')) . ' '
   \   . ' -'
   \ , ale_linters#c#gcc#GetCommand(bufnr(''))
 
@@ -53,7 +53,7 @@ Execute(The C GCC handler should include 'include' directories for projects with
   \ ale#Escape('gcc')
   \   . ' -S -x c -fsyntax-only '
   \   . '-iquote ' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/configure_project/subdir')) . ' '
-  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/configure_project') . '/include') . ' '
+  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/configure_project/include')) . ' '
   \   . ' -'
   \ , ale_linters#c#gcc#GetCommand(bufnr(''))
 
@@ -92,7 +92,7 @@ Execute(The C Clang handler should include 'include' directories for projects wi
   \   ale#Escape('clang')
   \   . ' -S -x c -fsyntax-only '
   \   . '-iquote ' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/makefile_project/subdir')) . ' '
-  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/makefile_project') . '/include') . ' '
+  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/makefile_project/include')) . ' '
   \   . ' -'
   \ , ale_linters#c#clang#GetCommand(bufnr(''))
 
@@ -144,7 +144,7 @@ Execute(The C++ GCC handler should include 'include' directories for projects wi
   \   ale#Escape('gcc')
   \   . ' -S -x c++ -fsyntax-only '
   \   . '-iquote ' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/makefile_project/subdir')) . ' '
-  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/makefile_project') . '/include') . ' '
+  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/makefile_project/include')) . ' '
   \   . ' -'
   \ , ale_linters#cpp#gcc#GetCommand(bufnr(''))
 
@@ -157,7 +157,7 @@ Execute(The C++ GCC handler should include 'include' directories for projects wi
   \   ale#Escape('gcc')
   \   . ' -S -x c++ -fsyntax-only '
   \   . '-iquote ' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/configure_project/subdir')) . ' '
-  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/configure_project') . '/include') . ' '
+  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/configure_project/include')) . ' '
   \   . ' -'
   \ , ale_linters#cpp#gcc#GetCommand(bufnr(''))
 
@@ -196,7 +196,7 @@ Execute(The C++ Clang handler should include 'include' directories for projects
   \   ale#Escape('clang++')
   \   . ' -S -x c++ -fsyntax-only '
   \   . '-iquote ' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/makefile_project/subdir')) . ' '
-  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/makefile_project') . '/include') . ' '
+  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/makefile_project/include')) . ' '
   \   . ' -'
   \ , ale_linters#cpp#clang#GetCommand(bufnr(''))
 
@@ -209,7 +209,7 @@ Execute(The C++ Clang handler should include 'include' directories for projects
   \   ale#Escape('clang++')
   \   . ' -S -x c++ -fsyntax-only '
   \   . '-iquote ' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/configure_project/subdir')) . ' '
-  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/configure_project') . '/include') . ' '
+  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/configure_project/include')) . ' '
   \   . ' -'
   \ , ale_linters#cpp#clang#GetCommand(bufnr(''))
 
@@ -256,7 +256,7 @@ Execute(The C++ Clang handler shoud use the include directory based on the .git
   \   ale#Escape('clang++')
   \   . ' -S -x c++ -fsyntax-only '
   \   . '-iquote ' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/git_and_nested_makefiles/src')) . ' '
-  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/git_and_nested_makefiles') . '/include') . ' '
+  \   . ' -I' . ale#Escape(ale#path#Winify(g:dir .  '/test_c_projects/git_and_nested_makefiles/include')) . ' '
   \   . ' -'
   \ , ale_linters#cpp#clang#GetCommand(bufnr(''))
 
@@ -268,7 +268,7 @@ Execute(The C++ ClangTidy handler should include json folders for projects with
   AssertEqual
   \ ale#Escape('clang-tidy')
   \   . ' -checks=' . ale#Escape('*') . ' %s '
-  \   . '-p ' . ale#Escape(ale#path#Winify(g:dir . '/test_c_projects/json_project') . '/build')
+  \   . '-p ' . ale#Escape(ale#path#Winify(g:dir . '/test_c_projects/json_project/build'))
   \ , ale_linters#cpp#clangtidy#GetCommand(bufnr(''))
 
 Execute(Move .git/HEAD back):
diff --git a/test/test_get_abspath.vader b/test/test_get_abspath.vader
index 5f813804..7e1b5930 100644
--- a/test/test_get_abspath.vader
+++ b/test/test_get_abspath.vader
@@ -1,15 +1,29 @@
 Execute(Relative paths should be resolved correctly):
   AssertEqual
-  \ '/foo/bar/baz/whatever.txt',
+  \ has('win32') ? '\foo\bar\baz\whatever.txt' : '/foo/bar/baz/whatever.txt',
   \ ale#path#GetAbsPath('/foo/bar/xyz', '../baz/whatever.txt')
   AssertEqual
-  \ has('win32') ? '/foo/bar/xyz\whatever.txt' : '/foo/bar/xyz/whatever.txt',
+  \ has('win32') ? '\foo\bar\xyz\whatever.txt' : '/foo/bar/xyz/whatever.txt',
   \ ale#path#GetAbsPath('/foo/bar/xyz', './whatever.txt')
   AssertEqual
-  \ has('win32') ? '/foo/bar/xyz\whatever.txt' : '/foo/bar/xyz/whatever.txt',
+  \ has('win32') ? '\foo\bar\xyz\whatever.txt' : '/foo/bar/xyz/whatever.txt',
   \ ale#path#GetAbsPath('/foo/bar/xyz', 'whatever.txt')
 
+  if has('win32')
+    AssertEqual
+    \ 'C:\foo\bar\baz\whatever.txt',
+    \ ale#path#GetAbsPath('C:\foo\bar\baz\xyz', '../whatever.txt')
+  endif
+
 Execute(Absolute paths should be resolved correctly):
   AssertEqual
-  \ '/ding/dong',
+  \ has('win32') ? '\ding\dong' : '/ding/dong',
   \ ale#path#GetAbsPath('/foo/bar/xyz', '/ding/dong')
+
+  AssertEqual
+  \ has('win32') ? '\ding\dong' : '/ding/dong',
+  \ ale#path#GetAbsPath('/foo/bar/xyz', '//ding/dong')
+
+  if has('win32')
+    AssertEqual '\ding', ale#path#GetAbsPath('/foo/bar/xyz', '\\ding')
+  endif
diff --git a/test/test_path_upwards.vader b/test/test_path_upwards.vader
index 8b81a109..cd461a28 100644
--- a/test/test_path_upwards.vader
+++ b/test/test_path_upwards.vader
@@ -1,52 +1,48 @@
-After:
-  let g:ale_has_override = {}
+Execute(ale#path#Upwards should return the correct path components):
+  if has('unix')
+    " Absolute paths should include / on the end.
+    AssertEqual
+    \ ['/foo/bar/baz', '/foo/bar', '/foo', '/'],
+    \ ale#path#Upwards('/foo/bar/baz')
+    AssertEqual
+    \ ['/foo/bar/baz', '/foo/bar', '/foo', '/'],
+    \ ale#path#Upwards('/foo/bar/baz///')
+    " Relative paths do not.
+    AssertEqual
+    \ ['foo/bar/baz', 'foo/bar', 'foo'],
+    \ ale#path#Upwards('foo/bar/baz')
+    AssertEqual
+    \ ['foo2/bar', 'foo2'],
+    \ ale#path#Upwards('foo//..////foo2////bar')
+    " Expect an empty List for empty strings.
+    AssertEqual [], ale#path#Upwards('')
+  endif
 
-Execute(ale#path#Upwards should return the correct path components for Unix):
-  let g:ale_has_override = {'win32': 0}
-
-  " Absolute paths should include / on the end.
-  AssertEqual
-  \ ['/foo/bar/baz', '/foo/bar', '/foo', '/'],
-  \ ale#path#Upwards('/foo/bar/baz')
-  AssertEqual
-  \ ['/foo/bar/baz', '/foo/bar', '/foo', '/'],
-  \ ale#path#Upwards('/foo/bar/baz///')
-  " Relative paths do not.
-  AssertEqual
-  \ ['foo/bar/baz', 'foo/bar', 'foo'],
-  \ ale#path#Upwards('foo/bar/baz')
-  AssertEqual
-  \ ['foo2/bar', 'foo2'],
-  \ ale#path#Upwards('foo//..////foo2////bar')
-  " Expect an empty List for empty strings.
-  AssertEqual [], ale#path#Upwards('')
-
-Execute(ale#path#Upwards should return the correct path components for Windows):
-  let g:ale_has_override = {'win32': 1}
-
-  AssertEqual
-  \ ['C:\foo\bar\baz', 'C:\foo\bar', 'C:\foo', 'C:\'],
-  \ ale#path#Upwards('C:\foo\bar\baz')
-  AssertEqual
-  \ ['C:\foo\bar\baz', 'C:\foo\bar', 'C:\foo', 'C:\'],
-  \ ale#path#Upwards('C:\foo\bar\baz\\\')
-  AssertEqual
-  \ ['/foo\bar\baz', '/foo\bar', '/foo', '/'],
-  \ ale#path#Upwards('/foo/bar/baz')
-  AssertEqual
-  \ ['foo\bar\baz', 'foo\bar', 'foo'],
-  \ ale#path#Upwards('foo/bar/baz')
-  AssertEqual
-  \ ['foo\bar\baz', 'foo\bar', 'foo'],
-  \ ale#path#Upwards('foo\bar\baz')
-  " simplify() is used internally, and should sort out \ paths when actually
-  " running Windows, which we can't test here.
-  AssertEqual
-  \ ['foo2\bar', 'foo2'],
-  \ ale#path#Upwards('foo//..///foo2////bar')
-  " Expect an empty List for empty strings.
-  AssertEqual [], ale#path#Upwards('')
-  " Paths starting with // return /
-  AssertEqual
-  \ ['/foo2\bar', '/foo2', '/'],
-  \ ale#path#Upwards('//foo//..///foo2////bar')
+  if has('win32')
+    AssertEqual
+    \ ['C:\foo\bar\baz', 'C:\foo\bar', 'C:\foo', 'C:\'],
+    \ ale#path#Upwards('C:\foo\bar\baz')
+    AssertEqual
+    \ ['C:\foo\bar\baz', 'C:\foo\bar', 'C:\foo', 'C:\'],
+    \ ale#path#Upwards('C:\foo\bar\baz\\\')
+    AssertEqual
+    \ ['/foo\bar\baz', '/foo\bar', '/foo', '/'],
+    \ ale#path#Upwards('/foo/bar/baz')
+    AssertEqual
+    \ ['foo\bar\baz', 'foo\bar', 'foo'],
+    \ ale#path#Upwards('foo/bar/baz')
+    AssertEqual
+    \ ['foo\bar\baz', 'foo\bar', 'foo'],
+    \ ale#path#Upwards('foo\bar\baz')
+    " simplify() is used internally, and should sort out \ paths when actually
+    " running Windows, which we can't test here.
+    AssertEqual
+    \ ['foo2\bar', 'foo2'],
+    \ ale#path#Upwards('foo//..///foo2////bar')
+    " Expect an empty List for empty strings.
+    AssertEqual [], ale#path#Upwards('')
+    " Paths starting with // return /
+    AssertEqual
+    \ ['/foo2\bar', '/foo2', '/'],
+    \ ale#path#Upwards('//foo//..///foo2////bar')
+  endif